There are several manufacturing processes in wood industry and other industries wherein products manufactured need to be dried at some stage of the process. The products are typically sheet-like products, such as paper, plasterboard or veneer sheets, for example. In the following it is mainly referred to veneer production.
Veneers used in the plywood or laminated veneer lumber manufacturing process are dried using a drying apparatus in order to achieve the level of a moisture defined by the gluing requirements. Before the gluing, the moisture of the veneer may be, e.g., less than 10 percent in order to succeed in the gluing process. Too high moisture may cause delamination in the glue line, because the high steam pressure prevents the formation of the glue line and causes eruption of the steam.
The veneer sheets may be dried using, e.g., sun drying or contact drying. Generally, in industrial manufacture of the plywood dryers based on convection heat transfer, such as roller dryer or screen dryer, are used. Roller dryer and screen dryer are similar to each other in terms of the air flow. In roller dryer the veneer sheets travel between rollers and the rollers are supported by a supporting structure. In the screen dryer, in turn, the veneer sheets travel between the screens reside above and below the veneer sheets and the screens are supported by the rollers. In both dryers air is used to transfer the heat so that the hot air is blown with circulation blowers against the veneer sheets by means of a jet box. The efficiency, i.e. drying capacity as well as energy efficiency, of the drying may be tuned by changing the temperature and/or moisture of the drying air. Typically air is heated by blowing it through the heat exchangers that are heated with thermal oil or steam or in some cases also with water and/or other heat transfer fluids. Alternatively or in addition, the dryer is heated with burners operating with a natural gas, butane, or heavy fuel oil and the flue gas of the burners mix with the air circulated by the circulation blower.
In modern dryers these blowers are mostly radial blowers. In some cases and in older dryer models axial blowers are commonly used Generally, a structure of a dryer includes a drying chamber having an input end and an output end and a conveyor that conveys the veneer sheets to be dried through the drying chamber. The chamber includes heating unit sections having at least one jet box to transfer the heat against the veneer sheets to be dried. A cooling section cools the veneer sheets leaving the output end of the drying chamber. Cooling is done to prevent too warm veneers to enter the lay-up line. If veneers are too warm, the glue applied onto to the veneer in lay-up line will dry out before the veneer lay-up is pre pressed and hot pressed. In modern veneer dyers the cooling section includes a pressure controller for maintaining the required pressure difference between the drying chamber and the cooling section.
The evenness of the drying result in the width direction of the dryer, i.e., in the longitudinal direction of the jet box, may be affected, e.g., with the shaping of the jet box. The jet box may be cone-shaped in the longitudinal direction in order to achieve even drying result along the width of the whole dryer. In this manner the pressure inside the jet box may be arranged as constant as possible along the width of the whole dryer and the air flow from each jet nozzle of the jet box may be retained as similar with each other as possible. The efficiency and the evenness of the heat transfer may be tuned by changing, e.g., the conicity of the jet box, the size of the jet nozzle, the shape of the jet nozzle, and/or the distance between the jet nozzles.
Generally speaking, the more even and efficient the heat transfer is, the more effective and efficient the drying is. Improving the heat transfer allows using smaller dryers to get the same production volume than with bigger dryers or improves the production volumes of the similar size dryer compared to the dryer with lower heat transfer capacity. The enhancing of the heat transfer reduces also the characteristic electric energy consumption, because less air circulation is needed to achieve the same heat transfer. This also applies to moisture transfer.
As mentioned above, the shape of the jet nozzle affects the efficiency of the jet box. A simple solution to realize the jet nozzle is to use a simple opening, but it is not the most efficient way. Thus, several different shapes of jet nozzles have been established and FIGS. 1a-1e illustrate some example solutions of the prior art jet nozzles such as flat opening (FIG. 1a), fingernail opening (FIG. 1b), flat slot opening (FIG. 1c), arc style opening (FIG. 1d), and orifice profile (FIG. 1e).
One drawback of the prior art solutions is that the jet nozzle may not be able to guide the air flow direction efficiently enough, thus the longitudinal incoming air flow is turned obliquely in respect to the incoming air flow and the surface of the veneer sheet. The same challenge also exists in a production of other sheet-like products, which needs to be dried. When the jet nozzle turns the air flow obliquely in respect to the incoming air flow and the surface of the veneer sheet, the guided air flows of the sequential jet nozzles in the longitudinal direction of the jet box may disturb each other, which in turn cause decreasing of the heat transfer.
Some of the prior art solutions could be improved to guide the air better, but that would cause more pressure loss in the jet nozzle. Increased pressure loss would mean need for more circulation blower power and need for higher pressures in the jet box. The higher power demand increases the electricity consumption and thus also the costs are increased. Hence, there is need to develop the existing solutions further in order to improve the efficiency of drying.